1 . Field of the Invention
This invention relates to the selective extraction of aromatic hydrocarbons from a bed stream comprising a mixture of aromatic and non-aromatic hydrocarbons by use of a selective extraction solvent and to the improvement of that process so as to produce increased yields of improved quality product.
2. Description of the Related Art
The separation of aromatics from hydrocarbon feed streams comprising mixtures of aromatics and non-aromatics by solvent extraction is a process which has long been practiced in the refining industry especially in the production of lubricating oil. The process involves the use of solvents such as phenol, furfural, n-methyl pyrrolidone which are selective for the aromatic components present in the hydrocarbon feed streams. The hydrocarbon stream and the selective solvents are combined, typically and preferably under counter-current conditions. The contacting results in concentration of the aromatic component in the selective solvent. Because the solvent and the hydrocarbon oil are of different densities and generally immiscible after the contacting, the aromatics rich solvent phase separates from the mixture thereby resulting in an aromatics rich solvent phase, called the extract, and an aromatics lean non-aromatics rich product phase called the raffinate. Because no solvent extraction process can be one hundred percent selective, the aromatics rich extract phase contains a minor but economically significant quantity of non-aromatic hydrocarbon which constitute good lube oil molecules.
Processes designed to recover these good lube oil molecules present in the extract phase have been proposed and disclosed in the literature.
U.S. Pat. No. 5,242,579 teaches a control method for solvent refining lube oils. The process disclosed consists of a solvent extraction zone wherein feed and solvent are contacted countercurrently. The primary extract is separated into a secondary raffinate and a secondary extract by cooling the primary extract 10.degree. F. to 120.degree. F. below the extraction temperature. The addition of an anti-solvent to the primary extract in combination with at least 10.degree. F. cooling is optional. Secondary raffinate is passed to the extraction zone with the fresh feed.
U.S. Pat. No. 4,328,092 teaches the solvent extraction of hydrocarbon oils. The process uses N-methyl-2-pyrrolidone. The extract from the solvent extraction zone is cooled to form two immiscible liquid phases, a secondary extract phase and a secondary raffinate phase. The secondary raffinate phase is recycled to the extraction zone resulting in increased yield of refined oil product and in energy savings.
U.S. Pat. No. 4,311,583 teaches a solvent extraction process. A hydrocarbon feed is contacted with N-methyl pyrrolidone in an extraction zone. The primary extract is separated into a secondary raffinate and a secondary extract by cooling the primary extract optionally with the addition of water or wet solvent. The secondary raffinate is separated from the secondary extract. At least part of the secondary raffinate is combined with the raffinate to obtain an increased yield of desired quality raffinate oil product. As part of the secondary raffinate may be returned to the lower part of the extraction zone.
French Patent 2,595,371 teaches a process for the selective solvent extraction of a hydrocarbon mixture. Solvent is passed counter-currently to the hydrocarbon feed employing 2 or more separation columns resulting in the separation of the feed into a raffinate, a pseudo-raffinate and an extract. Feed is introduced into a first column while fresh solvent is introduced into the top of a second column. The overheads from the first column constitute the feed to the second column. The bottoms from the second column are cooled and permitted to salt-out in a separation zone wherein an upper phase pseudo raffinate is recovered and a bottom phase of recycle solvent is recovered. This bottom phase recycle solvent is used as the solvent introduced into the first column. Extract is recovered from the bottom of the first column and raffinate from the top of the second column. In an alternative embodiment part of the pseudo raffinate can be cycled back to the bottom of the second column while the extract from the first column can be cooled to salt-out in a separation zone producing an upper phase of lighter hydrocarbon which is recycled to the bottom of the first column, and a true extract bottoms phase.
French Patent 2,595,370 teaches a multiple effect extraction process using counter-current solvent flow. The process utilizes a main column separated into 2 zones by a draw-off tray and a second column which fractionates the side stream drawn off from the first column. The fractionation zone produces an overhead raffinate which is fed back to the top zone of column 1 above the draw-off tray. The bottoms from the fractionation zone are cooled and separated into a pseudo raffinate and an extract. This extract is recycled to the bottom zone of column 1 just below the draw-off tray. It can optionally also be fed into the top zone of column 1 just above the draw-off tray. By this scheme a raffinate is recovered from the top of the first column, an extract from the bottom of said column and a pseudo raffinate from the separation zone to which the bottoms fraction from the fractionation zone is fed.
In an alternate embodiment the extract from the bottom of column 1 can be cooled to salt-out in a separation zone an upper phase of lighter hydrocarbons which is recycled back to the bottom of the bottom zone of column 1. The bottoms fraction from this separation zone is a true extract phase.